This application claims the priority of German patent 198 04 458.5-12, filed Feb. 5, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an arrangement and a method for detecting a fatigue fracture of a bolted connection caused by overstressing.
An arrangement of this type is known from commonly-owned U.S. patent application Ser. No. 08/929,259 (counterpart of German Patent Document DE 196 37 592 C1), the disclosure of which is expressly incorporated by reference herein. Because of the special construction and division of the bolted connection into an area which axially and radially has play and into an area which has no play, a predetermined breaking point is created at the transition between the areas which, after a defined number of overstressing situations exercised on the bolted connection will break as the result of material fatigue or during an excessive overstressing, whereby the resulting crack extends in a locally defined manner transversely to the longitudinal course of the bolted connection, thus in the circumferential direction. If the area without play starts from the bolt head and the dimensions of the bolt are designed correspondingly in comparison to the hollow section, the hollow section will break while a still sufficiently supporting residual joining connection remains between the bolt and the hollow section, the flux of force of the overstressing taking place only by way of the now supporting area which has play. However, sometimes the crack is not clearly visible with the naked eye and is therefore not obvious so that no close examination is made. This examination will only take place during a regular inspection, during which a possible crack is detected by use of suitable detection apparatuses, for example, on an ultrasound basis or by measuring the conducting capacity. However, in this case, the finding of the crack is facilitated by means of the site of the predetermined breaking point known a priori.
It is an object of the invention to further develop an arrangement of the above-mentioned type such that the occurrence of the crack on the hollow section of the bolted connection in the case of a fracture caused by fatigue can be detected in a simple manner at any time.
According to the invention, this and other objects have been achieved by providing an arrangement for detecting a fatigue fracture due to alternate axial stress in a bolted connection, comprising: a hollow section having an internal thread; a bolt with an external thread being threaded into said internal thread of said hollow section over an axial distance, said axial distance including a rearward area extending from a connection end of the bolt with play between said internal thread and said external thread, and a forward area adjacent to said rearward area without play between said internal thread and said external thread, said rearward area and said forward area defining a predetermined breaking point therebetween at which said hollow section will crack in a transverse direction due to fatigue from said alternate axial stress, said predetermined breaking point being located such that the bolt continues to support the hollow section at the rearward area after said hollow section cracks; and a signaling device arranged adjacent said predetermined breaking point, said signaling device triggering at least one signal which can be noticed visually or acoustically when said hollow section cracks.
According to the invention, the crack of the hollow section can be easily detected because the observer receives visually and/or acoustically noticeable signals which draw attention to the crack. The device which automatically triggers these signals simulates the effect of the overstressing existing in the case of a material fatigue, which is exhibited in the crack of the hollow section, in parallel thereto or shortly after the crack formation. Due to the signaling device's contact with the two areas and because it extends in both areas, it is destroyed by the crack and, when it is arranged only in the area of the residual joining connection, it is destroyed by the stressing of the bolted connection displaced to the area with play. When the device is destroyed, it causes the emission of the above-mentioned signals. The device may have any conceivable simple design. It must only be destructible and be able to transmit a signal at the point in time of its destruction which comes to the attention of the person using the article containing the bolted connection. A display of the signal may take place by way of the device itself or by a separate display device, whose manufacturing results in only low expenditures.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.